Assembly Language Fundamental- Computer Organisation

Assembly Language for Intel-
Based Computers,
Assembly Language
Fundamentals

Web site Examples
 Basic Elements of Assembly Language
 Example: Adding and Subtracting Integers
 Assembling, Linking, and Running Programs
 Defining Data
 Symbolic Constants
 Real-Address Mode Programming
2
Chapter Overview

Web site Examples
 Integer constants
 Integer expressions
 Character and string constants
 Reserved words and identifiers
 Directives and instructions
 Labels
 Mnemonics and Operands
 Comments
 Examples
3
Basic Elements of Assembly
Language

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 Optional leading + or – sign
 binary, decimal, hexadecimal, or octal digits
 Common radix characters:
◦ h – hexadecimal
◦ d – decimal
◦ b – binary
◦ r – encoded real
Examples: 30d, 6Ah, 42, 1101b
Hexadecimal beginning with letter: 0A5h
4
Integer Constants

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 Operators and precedence levels:
 Examples:
5
Integer Expressions

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 Enclose character in single or double quotes
◦ 'A', "x"
◦ ASCII character = 1 byte
 Enclose strings in single or double quotes
◦ "ABC"
◦ 'xyz'
◦ Each character occupies a single byte
 Embedded quotes:
◦ 'Say "Goodnight, Gracie” '
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Character and String
Constants

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 Reserved words cannot be used as identifiers
◦ Instruction mnemonics, directives, type attributes,
operators, predefined symbols
◦ See MASM reference in Appendix A
 Identifiers
◦ 1-247 characters, including digits
◦ not case sensitive
◦ first character must be a letter, _, @, ?, or $
7
Reserved Words and
Identifiers

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 Commands that are recognized and acted upon by the
assembler
◦ Not part of the Intel instruction set
◦ Used to declare code, data areas, select memory
model, declare procedures, etc.
◦ not case sensitive
◦ For example: .data,.code,.stack, DWORD,BYTE
 Different assemblers have different directives
◦ NASM not the same as MASM
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Directives

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 Assembled into machine code by assembler
 Executed at runtime by the CPU
 We use the Intel IA-32 instruction set
 An instruction contains:
 Label (optional)
 Mnemonic (required)
 Operand (depends on the instruction)
 Comment (optional)
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Instructions

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 Act as place markers
◦ marks the address (offset) of code and data
 Follow identifier rules
 Data label
◦ must be unique
◦ example: myArray (not followed by colon)
 Code label
◦ target of jump and loop instructions
◦ example:mov ax,bx
jmp target
target: add cx,ax (followed by colon)
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Labels

Web site Examples
 Instruction Mnemonics
◦ memory aid
◦ examples: MOV, ADD, SUB, MUL, INC, DEC
 Operands
◦ constant
◦ constant expression
◦ register
◦ memory (data label)
Constants and constant expressions are often called
immediate values
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Mnemonics and Operands

Web site Examples
 Comments are good!
◦ explain the program's purpose
◦ when it was written, and by whom
◦ revision information
◦ tricky coding techniques
◦ application-specific explanations
 Single-line comments
◦ begin with semicolon (;)
 Multi-line comments
◦ begin with COMMENT directive and a programmer-
chosen character
◦ end with the same programmer-chosen character
12
Comments

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 This is a comment;
 COMMENT ! This is comment 1
This is comment 2
This is comment 3
!
13
Eg: comments

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 No operands
 stc ; set Carry flag
 One operand
 inc eax ; register
 inc myByte ; memory
 Two operands
 add ebx,ecx ; register, register
 sub myByte,25 ; memory, constant
 add eax,36 * 25 ; register, constant-
expression
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Instruction Format Examples

Web site Examples
Mnemonic destination, source
 add ebx, ecx ; register, register
 Add 3, 5 ; 8:5+3
 Mov eax, 3
 Mov ebx, 5
 Add ebx, eax
 sub myByte,25 ; memory, constant
 add eax,36 * 25 ; register, constant-expression
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Syntax for operand
data is copied from source to destination

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 Defining Data
16
What's Next

Web site Examples
Example: Adding and Subtracting
Integers
17
TITLE Add and Subtract (AddSub.asm)
; This program adds and subtracts 32-bit integers.
INCLUDE Irvine32.inc
.code
main PROC
mov eax,10000h ; EAX = 10000h
add eax,40000h ; EAX = 50000h
sub eax,20000h ; EAX = 30000h
call DumpRegs ; display registers
exit
main ENDP
END main

Web site Examples
Program explanation
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 TITLE= Marks the entire lines as a comment
 INCLUDE= copies necessary definition and setup information from
a text file name Irvine32.inc (located in assembler’s INCLUDE
directory)
 .code = beginning of the code segment, executable statements
located
 PROC= beginning of a procedure
 MOV= copies from source to destination
 ADD/SUB = operator
 CALL = call a procedure, display current value in CPU registers
 EXIT= halts the program
 ENDP=end the main procedure
 END = marks the last line

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Example Output
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Program output, showing registers and flags:
EAX=00030000 EBX=7FFDF000 ECX=00000101 EDX=FFFFFFFF
ESI=00000000 EDI=00000000 EBP=0012FFF0 ESP=0012FFC4
EIP=00401024 EFL=00000206 CF=0 SF=0 ZF=0 OF=0
EIP= extended instruction pointer
EPL= extended flags

Web site Examples
 Some approaches to capitalization
 capitalize nothing
 capitalize everything
 capitalize all reserved words, including
instruction mnemonics and register names
 capitalize only directives and operators
 Other suggestions
 descriptive identifier names
 spaces surrounding arithmetic operators
 blank lines between procedures
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Suggested Coding Standards (1 of 2)

Web site Examples
 Indentation and spacing
◦ code and data labels – no indentation
◦ executable instructions – indent 4-5 spaces
◦ comments: begin at column 40-45, aligned
vertically
◦ 1-3 spaces between instruction and its operands
 ex: mov ax,bx
◦ 1-2 blank lines between procedures
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Suggested Coding Standards (2 of 2)

Web site Examples
Alternative Version of AddSub
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TITLE Add and Subtract (AddSubAlt.asm)
.386
.MODEL flat,stdcall
.STACK 4096
ExitProcess PROTO, dwExitCode:DWORD
DumpRegs PROTO
.code
main PROC
mov eax,10000h ; EAX = 10000h
add eax,40000h ; EAX = 50000h
sub eax,20000h ; EAX = 30000h
call DumpRegs
INVOKE ExitProcess,0
main ENDP
END main

Web site Examples
23
TITLE Add and Subtract (AddSubAlt.asm)
.386 = minimum CPU required (Intel386)
.MODEL flat,stdcall
.MODEL =instruct assmb to generate code
for protected mode;
STDCALL= calling MS-Windows functions
.STACK 4096
ExitProcess PROTO, dwExitCode:DWORD
DumpRegs PROTO
PROTO = declare prototypes for procedure used by the prog
ExitProcess= MS-Windows Function halts the current prog
DumpRegs= prcedure form Irvine32.inc to display registers
.code
main PROC
mov eax,10000h ; EAX = 10000h
add eax,40000h ; EAX = 50000h
sub eax,20000h ; EAX = 30000h
call DumpRegs
INVOKE ExitProcess,0
INVOKE= directive to call procedure or function
ExitProcess=passing it a return code of zero
main ENDP
END main

Web site Examples
Program Template
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TITLE Program Template (Template.asm)
; Program Description:
; Author:
; Creation Date:
; Revisions:
; Date: Modified by:
.data
; (insert variables here)
.code
main PROC
; (insert executable instructions here)
exit
main ENDP
; (insert additional procedures here)
END main
Instructors: please
customize as needed

Web site Examples
 Defining Data
25
What's Next

Web site Examples
 Assemble-Link-Execute Cycle
 make32.bat
 Listing File
 Map File
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Assembling, Linking, and Running
Programs

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 The following diagram describes the steps from creating a
source program through executing the compiled program.
 If the source code is modified, Steps 2 through 4 must be
repeated.
27
Assemble-Link Execute Cycle

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 Text editor= create an ASCII text file name source file
 Assembler= reads the source file, produce object file (a
machine language translation); produce listing file
(optionally)
 Linker=
◦ reads the object file and check call for procedure in a link
library
◦ copies any required procedure form library
◦ Combine procedure with object file, produce executable file
◦ Produce map file
 Loader= reads the executable file into memory and
branches the CPU to prog starting address
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Summaries

Web site Examples
 Use it to see how your program is compiled
 Contains
◦ source code
◦ addresses
◦ object code (machine language)
◦ segment names
◦ symbols (variables, procedures, and constants)
 Example: addSub.lst
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Listing File

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 Information about each program segment:
 starting address
 ending address
 size
 segment type
 Example: addSub.map (16-bit version)
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Map File

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 Defining Data
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What's Next

Web site Examples
 Intrinsic Data Types
 Data Definition Statement
 Defining BYTE and SBYTE Data
 Defining WORD and SWORD Data
 Defining DWORD and SDWORD Data
 Defining QWORD Data
 Defining TBYTE Data
 Defining Real Number Data
 Little Endian Order
 Adding Variables to the AddSub Program
 Declaring Uninitialized Data
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Defining Data

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 BYTE, SBYTE
◦ 8-bit unsigned integer; 8-bit signed integer
 WORD, SWORD
◦ 16-bit unsigned & signed integer
 DWORD, SDWORD
◦ 32-bit unsigned & signed integer
 QWORD
◦ 64-bit integer
 TBYTE
◦ 80-bit integer
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Intrinsic Data Types (1 of 2)

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 REAL4
 4-byte IEEE short real
 REAL8
 8-byte IEEE long real
 REAL10
 10-byte IEEE extended real
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Intrinsic Data Types (2 of 2)

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 A data definition statement sets aside storage in
memory for a variable.
 May optionally assign a name (label) to the data
 Syntax:
[name] directive initializer [,initializer] . . .
value1 BYTE 10
◦ All initializers become binary data in memory
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Data Definition Statement

Web site Examples
Defining BYTE and SBYTE Data
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value1 BYTE 'A' ; character constant
value2 BYTE 0 ; smallest unsigned byte
value3 BYTE 255 ; largest unsigned byte
value4 SBYTE -128 ; smallest signed byte
value5 SBYTE +127 ; largest signed byte
value6 BYTE ? ; uninitialized byte
Each of the following defines a single byte of storage:
• MASM does not prevent you from initializing a BYTE with a
negative value, but it's considered poor style.
• If you declare a SBYTE variable, the Microsoft debugger will
automatically display its value in decimal with a leading sign.

Web site Examples
Defining Byte Arrays
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list1 BYTE 10,20,30,40
list2 BYTE 10,20,30,40
BYTE 50,60,70,80
BYTE 81,82,83,84
list3 BYTE ?,32,41h,00100010b
list4 BYTE 0Ah,20h,‘A’,22h
Examples that use multiple initializers:

Web site Examples
 A string is implemented as an array of characters
◦ For convenience, it is usually enclosed in quotation marks
◦ It often will be null-terminated
 Examples:
38
Defining Strings (1 of 3)
str1 BYTE "Enter your name",0
str2 BYTE 'Error: halting program',0
str3 BYTE 'A','E','I','O','U'
greeting BYTE "Welcome to the Encryption Demo program "
BYTE "created by Kip Irvine.",0

Web site Examples
 To continue a single string across multiple lines, end
each line with a comma:
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menu BYTE "Checking Account",0dh,0ah,0dh,0ah,
"1. Create a new account",0dh,0ah,
"2. Open an existing account",0dh,0ah,
"3. Credit the account",0dh,0ah,
"4. Debit the account",0dh,0ah,
"5. Exit",0ah,0ah,
"Choice> ",0

Web site Examples
 End-of-line character sequence:
◦ 0Dh = carriage return
◦ 0Ah = line feed
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str1 BYTE "Enter your name: ",0Dh,0Ah
BYTE "Enter your address: ",0
newLine BYTE 0Dh,0Ah,0
Idea: Define all strings used by your program in the same
area of the data segment.

Web site Examples
 Use DUP to allocate (create space for) an array or
string. Syntax: counter DUP ( argument )
 Counter and argument must be constants or
constant expressions
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Using the DUP Operator
var1 BYTE 20 DUP(0) ; 20 bytes, all equal to zero
var2 BYTE 20 DUP(?) ; 20 bytes, uninitialized
var3 BYTE 4 DUP("STACK") ; 20 bytes: "STACKSTACKSTACKSTACK"
var4 BYTE 10,3 DUP(0),20 ; 5 bytes

Web site Examples
 Define storage for 16-bit integers
◦ or double characters
◦ single value or multiple values
42
Defining WORD and SWORD Data
word1 WORD 65535 ; largest unsigned value
word2 SWORD –32768 ; smallest signed value
word3 WORD ? ; uninitialized, unsigned
word4 WORD "AB" ; double characters
myList WORD 1,2,3,4,5 ; array of words
array WORD 5 DUP(?) ; uninitialized array

Web site Examples
Defining DWORD and SDWORD Data
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val1 DWORD 12345678h ; unsigned
val2 SDWORD –2147483648 ; signed
val3 DWORD 20 DUP(?) ; unsigned array
val4 SDWORD –3,–2,–1,0,1 ; signed array
Storage definitions for signed and unsigned 32-bit
integers:

Web site Examples
Defining QWORD, TBYTE, Real
Data
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quad1 QWORD 1234567812345678h
val1 TBYTE 1000000000123456789Ah
rVal1 REAL4 -2.1
rVal2 REAL8 3.2E-260
rVal3 REAL10 4.6E+4096
ShortArray REAL4 20 DUP(0.0)
Storage definitions for quadwords, tenbyte values,
and real numbers:

Web site Examples
 All data types larger than a byte store their individual
bytes in reverse order. The least significant byte occurs at
the first (lowest) memory address.
 Example:
val1 DWORD 12345678h
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Little Endian Order

Web site Examples
Adding Variables to AddSub
46
TITLE Add and Subtract, Version 2 (AddSub2.asm)
; This program adds and subtracts 32-bit unsigned
; integers and stores the sum in a variable.
.data
val1 DWORD 10000h
val2 DWORD 40000h
val3 DWORD 20000h
finalVal DWORD ?
.code
main PROC
mov eax,val1 ; start with 10000h
add eax,val2 ; add 40000h
sub eax,val3 ; subtract 20000h
mov finalVal,eax ; store the result (30000h)
call DumpRegs ; display the registers
exit
main ENDP
END main

Web site Examples
 Use the .data? directive to declare an unintialized
data segment:
.data?
 Within the segment, declare variables with "?"
initializers:
smallArray DWORD 10 DUP(?)
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Declaring Unitialized Data
Advantage: the program's EXE file size is reduced.

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 Defining Data
48
What's Next

Web site Examples
 Equal-Sign Directive
 Calculating the Sizes of Arrays and Strings
 EQU Directive
 TEXTEQU Directive
49
Symbolic Constants

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 name = expression
 expression is a 32-bit integer (expression or
constant)
 may be redefined
 name is called a symbolic constant
 good programming style to use symbols
50
Equal-Sign Directive
COUNT = 500
.
.
mov al,COUNT

Web site Examples
 current location counter: $
 subtract address of list
 difference is the number of bytes
51
Calculating the Size of a Byte
Array
list BYTE 10,20,30,40
ListSize = ($ - list)

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Divide total number of bytes by 2 (the size of a word)
52
Calculating the Size of a Word
Array
list WORD 1000h,2000h,3000h,4000h
ListSize = ($ - list) / 2

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Divide total number of bytes by 4 (the size of a
doubleword)
53
Calculating the Size of a Doubleword
Array
list DWORD 1,2,3,4
ListSize = ($ - list) / 4

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 Define a symbol as either an integer or text expression.
 Cannot be redefined
54
EQU Directive
PI EQU <3.1416>
pressKey EQU <"Press any key to continue...",0>
.data
prompt BYTE pressKey

Web site Examples
 Define a symbol as either an integer or text expression.
 Called a text macro
 Can be redefined
55
TEXTEQU Directive
continueMsg TEXTEQU <"Do you wish to continue (Y/N)?">
rowSize = 5
.data
prompt1 BYTE continueMsg
count TEXTEQU %(rowSize * 2) ; evaluates the expression
setupAL TEXTEQU <mov al,count>
.code
setupAL ; generates: "mov al,10"

Web site Examples
 Basic Elements of Assembly Language
 Example: Adding and Subtracting Integers
 Assembling, Linking, and Running Programs
 Defining Data
 Symbolic Constants
 Real-Address Mode Programming
56
What's Next

Web site Examples
 Generate 16-bit MS-DOS Programs
 Advantages
 enables calling of MS-DOS and BIOS functions
 no memory access restrictions
 Disadvantages
 must be aware of both segments and offsets
 cannot call Win32 functions (Windows 95 onward)
 limited to 640K program memory
57
Real-Address Mode
Programming (1 of 2)

Web site Examples
 Requirements
 INCLUDE Irvine16.inc
 Initialize DS to the data segment:
mov ax,@data
mov ds,ax
58
Real-Address Mode Programming (2 of
2)

Web site Examples
Add and Subtract, 16-Bit
Version
59
TITLE Add and Subtract, Version 2 (AddSub2r.asm)
.data
val1 DWORD 10000h
val2 DWORD 40000h
val3 DWORD 20000h
finalVal DWORD ?
.code
main PROC
mov ax,@data ; initialize DS
mov ds,ax
mov eax,val1 ; get first value
add eax,val2 ; add second value
sub eax,val3 ; subtract third value
mov finalVal,eax ; store the result
call DumpRegs ; display registers
exit
main ENDP
END main

Web site Examples
 Integer expression, character constant
 directive – interpreted by the assembler
 instruction – executes at runtime
 code, data, and stack segments
 source, listing, object, map, executable files
 Data definition directives:
 BYTE, SBYTE, WORD, SWORD, DWORD, SDWORD,
QWORD, TBYTE, REAL4, REAL8, and REAL10
 DUP operator, location counter ($)
 Symbolic constant
 EQU and TEXTEQU
60
Summary

Assembly Language Fundamental- Computer Organisation

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